Bulk molding compound, or BMC, is a fibre reinforced thermoset molding compound that can be moulded without reaction byproducts under only enough pressure to flow and compact the material. BMC are used currently in high performance engineering thermoset molding compounds and used extensively in the electrical, automotive and consumer goods industries.
What is BMC formed?
Composite materials are obtained by mixing two materials to obtain a material with unique mechanical properties. This happens too with Bulk Molding Compound.
Resin
As show is this post about types of matrixes, for composite materials there are a lot of different types of matrixes to use. For Bulk Molding Compound the most used ones are polyesters, vinyl esters and epoxies, which are used to products that require higher strength properties.
Fibers
Glass fibre reinforcement is used to achieve necessary dimensional stability and mechanical properties. E-glass is the most common fibre reinforcement for composites. Glass loading normally averages 30% by weight in compression moulded composites but can vary from 18-65% by weight.
Fillers
Fillers are used to improve physical properties, reduce volumetric shrinkage of the resin and to reduce costs. They are typically divided into functional and non-functional categories. Examples of functional fillers include alumina trihydrate for flame retardancy, hollow glass bubbles for lower weights, mica and wollastonite for reinforcement. Non-functional fillers are used for cost reduction and are mineral based. Ground limestone (CaCO3,) is the most common type of filler.
Properties
Reinforced composites materials offer the maximum design versatility and capability of any material. With the excellent cost/performance characteristics of reinforced composites, the variety and quantity of products being produced with these materials grow annually around the world.
- Part consolidation: Reinforced composites can be molded in three dimensions in one operation. Complex shapes that require multi-piece assembly using materials, such as wood or steel, may be molded in one step with the use of ribs, bosses and varying wall thickness.
- Light weight: Reinforced composites offer a greater strength-to-weight ratio than most non-reinforced plastics and many metals.
- Dimensional stability: Reinforced composites can maintain the critical tolerances required of the most demanding applications. Composites meet the most stringent material stiffness, dimensional tolerance, weight and cost criteria in many diverse applications.
- High strength: Reinforced composites have excellent strength-to-weight properties. By weight, reinforced composites surpass the tensile strength of iron, carbon and stainless steels. Many glasses reinforced compounds equal or exceed the flexural strength and impact resistance of metals.
- Corrosion resistance: Reinforced composites do not rust or corrode, are resistant to attack from most organic chemicals and can be formulated to resist acidic and basic solutions.
- Electrical resistance: Reinforced composites are very poor conductors of electricity.
- Resistance to minor impact: Reinforced composite components have an excellent memory characteristic. Instead of yielding or deforming under minor impact as with steel, a reinforced composite panel will deflect and spring back to its original surface form.
- Surface quality: Reinforced composites can achieve a variety of surface textures, from very smooth and glossy to a rough texture. Insignias and alphanumeric characters can be molded as raised or indented characters.
- Molded-in color: Color can be added to the reinforced composite compound, often eliminating the need for a secondary painting process.
Design considerations for Bulk Moulding Compound
Given the wide range of options provided by reinforced composites, it is imperative that the designer accurately establish the functional and performance requirements of the product or component.
Draft
Draft is a slight angle introduced relative to the direction of the opening and closing of the mold. It is necessary to design the part so that all side walls, both interior and exterior, have draft. This enables the part to be removed from the mold without hanging or rubbing, which can degrade ‘appearance’ surfaces. Minimum draft angle of 1″ for the first 76.2 mm (3 in) of depth, 2″ for 76.2-101.6 mm (34 in) of depth, 3″ for 101.6-127 mm (4-6 in) of depth and 1″ for every additional 50.8 mm (2in) thereafter is recommended on all surfaces parallel to the mold movement.
Radius
In mold making, the radius defines the curvature established between two intersecting surfaces. The more generous the radius, the better the flow of molding material for a stronger part. A minimum radius of 1.59 mm (1/16 in) is recommended for all radii for both interior and exterior plane intersections. Radii should be designed to maintain relatively uniform part thickness. Ribs and bosses opposite an appearance surface should have the radii eliminated to reduce the likelihood of warpage or ’sink’ (surface depression).
Nominal thickness
The nominal thickness is the overall design thickness of most of the part. It is desirable to establish uniform thickness throughout a part, to achieve minimum cure time, uniform cooling and minimize warpage and shrinkage. Nominal thickness for reinforced composites is 2.544.57 mm (0.100-0.180 in). Recommended minimum thickness is 1.53 mm (0.060 in). Recommended maximum thickness 25.4 mm (1.00 in). By designing hollow ribs, bosses and elevation changes can achieve intricate part geometry while maintaining nominal thickness throughout the part.
Ribs
Linear projections 90″ from the plane surface of a part are called ribs. The use of ribs will allow the part to meet strength and rigidity requirements, preventing warpage and bowing in large plane surfaces while reducing the bulk and mass of a part. Ribs should be designed to maintain the nominal thickness and follow the guidelines.
Edge stiffening (a) Preferred edge flange designs to increase panel stiffness; (b) Thickening the edge flange may increase cycle times. for draft angles. They should be dimensioned so that their thickness at the juncture of the rib with its plane surface is between 75 and 90% of nominal.
Inserts
Inserts are objects (usually metal) which are molded into a part to facilitate repetitive fastening and unfastening of associated parts and can be provided with male or female threads. They can provide bearing or bushing surfaces, electrical or other mechanical connections. Inserts should have knurls, grooves or shoulders to lock them in place and should be located parallel to the direction of mold travel.
